For protection against fires, liquid tanks, particularly those containing flammable liquids, must be equipped with permanently installed fire protection systems. These systems substantially comprise pumps, reservoirs for foaming agents, admixing means, foam generators, means of initiation, and pipeline and distribution systems (cf. DIN 14 493, part 2).
There are foam discharging devices on the tanks which, in the event of a fire, introduce the foam into the interior of the tank. In order to prevent combustible vapors or liquids from the tank space from penetrating the pipeline system and flowing via the air intake opening of the foam generator out into the open atmosphere, the foam discharging devices are provided with so-called foam pots. These take the form of cylindrical vessels into which the foam line is introduced from the bottom; a knee pipe provided with a safety shut-off establishes the connection to the tank. The task of the foam pots is to close off the tank space from the foam line in the normal situation, but to clear access for the extinguishing agent when foaming of the interior of the tank is required. The safety shut-off of the foam pot must be corrosion-resistant with respect to the products which are stored in the tank and, when required, it must be possible for it to be easily destroyed or easily opened. Therefore, it is customary, as a safety shut-off, to use rupture disks of glass, i.e. a material which resists most chemicals stored in tanks but easily breaks.
Temperature differences which exist between the interior of the tank and the foam pot can result in condensation of liquid vapors in the knee pipe of the foam pot. If the condensates are corrosive, there is a risk that the knee pipe will be destroyed and the product stored in the tank will escape from the air intake opening of the foam pipes.
Even if the rupture disk is inset above the knee pipe using sealing cement with the aid of a ring construction, there is need for maintenance and repair work. Since the sealing cement is often not resistant to the products stored in the tank, the liquid vapor in the foam pot condenses and the stored product escapes. Moreover, the rupture disks of glass are fragile and can break in cases of temperature fluctuations owing to the different expansion coefficients of glass and metal.
Various attempts have already been made to eliminate the deficiencies indicated of the known devices. For instance, the foam pots have been produced from high grade steel instead of from normal grade galvanized steel. However, commercial considerations stand in the way of their widespread installation. The installation of knee pipes of high grade steel instead of normal grade steel in foam pots of customary steel has also not become established practice. In this respect, although the corrosion problems can be overcome by completely or partially changing the material, the difficulties in sealing off the tank space from the foam line are not overcome thereby. To achieve an increased seal tightness, therefore, both a holding means and a fastening ring for the rupture disk at the upper end of the knee pipe have been provided with an additional groove for receiving a seal of soft material. However, this design measure also did not result in the desired gas-tight seal.